The present invention relates to processing of materials, and more particularly to plasma processing.
Plasma processing is widely used to modify surface properties of materials. Thus, plasma is used in fabrication of integrated circuits to perform deposition, etch, cleaning, and rapid thermal anneal. Plasma-based surface processes are also used for hardening of surgical instruments and machine tools, and are used in aerospace, automotive, steel, biomedical, and toxic waste management industries. See, for example, M. A. Lieberman and A. J. Lichtenberg. xe2x80x9cPrinciples of Plasma Discharges and Materials Processingxe2x80x9d (1994), page 1.
For some applications there are unique advantages to etching a semiconductor wafer with plasma. For example, the backsides of semiconductor wafers are sometimes etched to make the wafers thinner after the components and circuitry have been fabricated on the frontside of the wafer. The wafer can then be separated into dice. Plasma etching is used for wafer thinning because other thinning techniques (e.g., grinding) create too much stress on the wafer and may damage the wafer.
A common goal in plasma processing is high throughput and high processing uniformity.
FIG. 1 shows a prior art plasma processing system 110 described in U.S. patent application Ser. No. 08/975,403 and PCT application WO 99/26796 which are incorporated herein by reference. Plasma source 114 generates a plasma jet 120 schematically shown by an arrow. Carrousel 124 has five wafer holders 130 (or some other number of wafer holders) each of which holds a semiconductor wafer. The wafers, not shown in FIG. 1, are positioned beneath the holders 130. Plasma jet 120 flows upwards and impinges on the wafers bottom surfaces. Holders 130 may be non-contact vortex holders (these holders do not contact the wafers top surface), or they may be contact holders that hold the wafers by vacuum or by electrostatic or mechanical means.
Plasma processing occurs at atmospheric pressure. Plasma jet 120 is too narrow to cover an entire wafer, so the wafers are moved in and out of the plasma in a predetermined pattern aimed at achieving uniform processing. Each holder 130 is rigidly attached to a respective arm 140A of an angle drive 140. Angle drive 140 rotates the wafers around a vertical axis 140X. Angle drive 140 has a body 140B rigidly attached to an arm 150A of an angle drive 150. Drive 150 rotates the arm around a vertical axis 150X. Control system 154 (e.g. a computer) controls the drives 140 and 150.
Plan view FIGS. 2A-2C illustrate the wafer path. Only one wafer 134 is shown for simplicity. For each position of arm 150A, wafers 134 sweep through a ring-shaped (donut-shaped) path 202 centered at axis 140X. The actual path swept by the wafers is not a ring since drive 150 is not stationary, but a ring is a fair approximation of the wafer path if angular velocity W1 of drive 150 is several times smaller than angular velocity W2 of drive 140.
Numeral 220 denotes a stationary horizontal line that intersects the axis 150X and the center of plasma jet 120. Angle "THgr" is the angle between the line 220 and the arm 150A.
In FIG. 2A. "THgr"=0. Axis 140X is in its farthest position from plasma 120. The arms 140A, 150A, and the distance between the center of plasma 120 and the axis 150X, are dimensioned so that at "THgr"=0 the wafers do not pass over the plasma. This eliminates plasma processing during wafer loading and unloading. (Wafer loading and unloading occur at "THgr"=0.)
In the example of FIGS. 2A, 2B, 2C, arm 150A rotates clockwise. In FIG. 2B, the angle "THgr" has increased to some value "THgr"1, and the outer edge 134F of wafer 134 has entered the plasma 120. (The xe2x80x9couter edgexe2x80x9d refers to the most distant edge from axis 140X.) As "THgr" continues to increase, the plasma processes wafer points closer and closer to axis 140X. In FIG. 2C, the plasma processes the wafer edge 134C closest to axis 140X ("THgr" is some value "THgr"2). When angle "THgr" is 180xc2x0, no plasma processing takes place.
As "THgr" increases from 180xc2x0 to 360xc2x0, the wafer path 202 returns to its position in FIG. 2A via a symmetric route. For each value "THgr"o between 180xc2x0 and 360xc2x0, the positions of ring 202 for "THgr"="THgr"o and "THgr"=360xc2x0xe2x88x92"THgr"o are symmetric to each other relative to line 220.
An advantage of the system of FIG. 1 is that there is no need to move the plasma source 114. (In some earlier systems, a single wafer was positioned at the location of axis 140X; the plasma source had to move towards and away from the axis 150X to process the whole wafer.)
To achieve uniform processing, the system of FIG. 1 attempts to make each point on the wafer pass through the plasma the same number of times and spend the same amount of time in the plasma. The velocity W1 of drive 150 varies so that the wafer points located farther from axis 140X spend about the same time in the plasma as the points closer to the axis 140X. The wafer passes multiple times over the plasma during each revolution of drive 150. The paths traced by the plasma on the wafer surface in consecutive revolutions of drive 140 overlap. The overlap is particularly desirable because the plasma jet 120 may have non-uniform heat distribution across the jet""s horizontal cross section.
It is desirable to further improve processing uniformity while maintaining high processing throughput.
In the system of FIG. 1, processing uniformity may suffer at the wafer edges due to unstable plasma behavior when the wafer enters and exits the plasma. Another reason why the processing uniformity may suffer is as follows. As the wafer moves through the plasma, the processing byproducts are generated at the bottom surface of the wafer. These byproducts may impede the wafer processing near the wafer edge exiting the plasma.
To improve the processing uniformity, one can change the direction of the W2 rotation during processing. This solution is described in U.S. patent application Ser. No. 09/315,122 filed May 19, 1999 by O. Siniaguine et al. and incorporated herein by reference. Disadvantageously, changing the direction of the W2 rotation tends to increase the processing time. It is therefore desirable not to change the direction of the W2 rotation, or at least to reduce the number of times that the direction of the W2 rotation is changed.
Another problem noted in the U.S. patent application Ser. No. 09/315,122 relates to different cooling times obtained for the wafer points at different distances from the axis 140X of drive 140. As illustrated in FIGS. 2A, 2B, and 2C, the entire wafer is processed during each half-revolution of drive 150. The wafer is processed once when xcex8 changes from 0 to 180xc2x0, and once when xcex8 changes from 180xc2x0 to 360xc2x0. Each point P on the wafer""s bottom surface is processed when xcex8 is at or near some value xcex8P. When 0 increases past the value xcex8P, the point P is moved out of the plasma and is therefore cooled. The point P does not re-enter the plasma until xcex8 reaches the value 360xc2x0xe2x88x92xcex8P in the next half-revolution of drive 150. Then the point P becomes processed again, and then is cooled again until the angle xcex8 becomes equal to xcex8P.
As shown in the U.S. patent application Ser. No. 09/315,122, the cooling times may be different for different points on the wafer. To equalize the cooling times, U.S. patent application Ser. No. 09/315,122 proposes to suppress plasma processing during one half of each revolution of drive 150. For example, plasma processing could take place only when xcex8 changes from 0xc2x0 to 180xc2x0, or only when xcex8 changes from 180xc2x0 to 360xc2x0. Disadvantageously, suppressing the plasma processing during one half of each revolution tends to increase processing time.
In some embodiments of the present invention, the wafer is subjected to a third rotation in addition to the rotation of drives 140 and 150. For example, the wafer can be rotated around its axis, or some other axis, simultaneously with being rotated by drives 140 and 150. The processing uniformity is improved because the processing byproducts affect the wafer processing more uniformly across the surface of the wafer. In addition, the cooling times for different points on the wafer surface also become more uniform. These advantages can be achieved without suppressing the wafer processing during one half of each revolution of drive 150, and without changing the direction of rotation of drive 140. The throughput is therefore increased. However, the direction of rotation may be changed, and the wafer processing may be suppressed during one half of each revolution of drive 150, if desired.
Another advantage obtained in some embodiments of the present invention is illustrated in FIGS. 3, 4. As shown in FIG. 3, each path 120P traced by the plasma on the wafer surface in the system of FIG. 1 during a single revolution of drive 140 is approximately an arc with a center at axis 140X. The path approximates the arc because the velocity W2 is greater than W1. If the plasma processing is temperature sensitive (a temperature sensitive etch, for example) the processed wafer may have grooves and ridges extending in the direction of arcs 120P.
In some embodiments of the present invention, the third wafer rotation causes the plasma paths on the wafer to become more varied (FIG. 4). The processing uniformity is therefore improved.
The invention is not limited to the embodiments described above. Some embodiments provide a method for processing an article with plasma, the method comprising:
(a) generating the plasma;
(b) moving the article as the article contacts the plasma, wherein a motion of the article comprises at least a first rotational motion, a second rotational motion, and a third rotational motion which occur simultaneously.
Some embodiments provide an apparatus for moving an article through plasma, the apparatus comprising:
a first arm rotatable around a first axis;
a second arm rotatably attached to the first arm to rotate an article around a second axis; and
a rotational mechanism for inducing a rotational motion of the article in addition to, and simultaneously with, the rotation of the first and second arms.
Some embodiments provide articles processed by methods of the present invention.
Other features and advantages of the invention are described below.